I think it was found around chs 14-19.
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The letter is of course significant because of its content, but it is perhaps more noteworthy because of the simple fact that it is written. Hassan is communicating with Amir on an equal level, something he could never have done when they were boys. Hosseini gives a nod here to the power of the written word, which endures and has an effect that transcends even death-after all, Hassan is long dead by the time Amir reads his correspondence. This also gives legitimacy to Amir because, coward though he may be, he is a writer, an ambassador of the written word.
The topics of secrets and family ties converge and come to a climax in Chapter Seventeen, when Rahim Khan finally reveals to Amir that Hassan was his half-brother. Amir is furious because suddenly the way he treated Hassan and Ali seems all the more wrong. The concept of "brother" is much stronger to him than the concept of "servant-best-friend" whereas Hassan had treated him like a brother no matter what. As Amir says, "Hassan had loved me once, loved me in a way that no one ever had or ever would again."